1. Field of the Invention
The invention relates to an improved substrate for supporting electronic circuitry such as pluralities of integrated circuit chips, ceramic chip packages, discrete components etc., and requiring high temperature "thick film" interconnections. More particularly, the invention relates to an improved copper ceramic substrate of a thin, structurally strong design having a thermal design optimized for high density applications.
2. Description of the Prior Art
High temperature eutectic bonds for ceramic to metal seals have been made in power tubes and power semiconductors for some time. Such seals have been described in U.S. Pat. Nos. 3,744,120 (Burgess et al) and 3,766,634 (Babcock et al). In the Babcock et al patent entitled "Method of Direct Bonding of Metals to Non-Metallic Substrates", the formation of single bonds between a metal and a ceramic using a copper oxygen eutectic are shown. In addition, two three layer bonds are shown. In one case, two metallic layers are formed over a metallic core. The latter arrangement is used in the cylindrical seals used in fabricating high frequency vacuum tubes. A third U.S. Pat. No. 3,944,430 to Cusano et al deals with a cylindrical housing for a semiconductor device in one embodiment in which a cylindrical ceramic is bonded between two metallic members. In a further embodiment, an electrical circuit board assembly is suggested for application as the substrate for an SCR pellet. In this substrate, the core of the substrate is a ceramic, the upper surface is largely covered with a patterned copper conductor and the lower surface is covered with an unpatterned copper layer, the latter layer being used for soft soldering the entire assembly to a heat sink.
The applicability of such teaching to larger substrates has been suggested in an article in the IEEE March 1982, pages 149-153 by Wittmer entitled "Mechanical Properties of Liquid Phase Bonded Ceramic Substrates". The IEEE article considers both symmetric and nonsymmetric arrangements of an experimental nature in which a central ceramic core is provided with a copper layer on the top and bottom to form a laminate structure. The practical substrates described are used for power electronics devices and for monolithic rotors for disk motors. In these practical constructions, the copper on at least one surface of the ceramic is segmented, precluding the mechanical design from achieving true symmetry.
Substrates designed for mounting plural chip carriers, discrete components and conductor runs have been formed in two principal ways. Where high temperature processing has not been required, polymeric printed circuit boards have been used. Where high temperature processing was desired, asymmetric copper ceramic substrates using copper ceramic eutectic bonds have been used. In the conventional asymmetric copper-ceramic arrangement, the thickness of the copper has been held to a small fraction of the thickness of the ceramic so as to minimize the bowing of the structure at room temperatures due to differential stresses between the copper and the ceramic. Both structures have been thicker than optimum for a given strength and rigidity, and the designs have been of less than optimum thermal design for high density applications.
The thermal design issue has become more critical as the density requirement has increased. Current thermal criteria establish levels of heat generation in each substrate and dictate that each substrate be suitable for stacked assembly in close proximity to like substrates in an appropriate chassis or cabinetry. In these applications both the cabinetry and the substrates must be designed to remove the generated heat for control of the temperature rise in the equipment.